Diurnal variations in plasma concentrations of neutral amino acids. 2) Effect of dietary protein intake on amino acid ratios.

The ratios of plasma tryptophan, tyrosine and phenilanine levels to the sum of other large neutral amino acids are reported. They tended to fall as the protein content of the diet was increased. The isoleucine ratios were not correlated with dietary protein content. Since diet-induced changes in plasma tryptophan and tyrosine ratios in animal are known to cause parallel alterations in brain tryptophan and tyrosine and levels, and thus in the rates of brain serotonin and catecholamines synthesis, out data suggest that the ingestion of carbohydrates and protein may also normally affect brain monoamine synthesis in humans.